1. Field of the Invention
The invention relates to articles formed by chemical vapor deposition and methods of forming such articles.
2. Description of Related Art
Chemical vapor deposition (CVD) techniques have been widely used to provide thin films and coatings of a variety of materials on various products. Typically, the process involves reacting vaporized or gaseous chemical precursors in the vicinity of a substrate to result in a material such as silicon carbide (SiC) depositing on the substrate. The deposition reaction is continued until the deposit reaches the desired thickness.
CVD techniques can be used to form relatively thin coatings on the surfaces of pre-existing articles; in this situation, the surface of the article forms the substrate. However, CVD techniques can also be adapted to produce articles that are formed from the deposited material. In this situation, the substrate upon which deposition occurs is a form or mandrel that provides an initial shape to the article. The article, which is removed after sufficient deposition has occurred, has a complementary surface that corresponds to the form or mandrel. Such articles are called “free-standing” articles herein.
One method by which free-standing SiC articles are formed by CVD includes feeding silicon carbide precursor gases or vapors into a deposition chamber, where they are heated to a temperature at which they react to produce silicon carbide. The precursor gases or vapors react at the surface of a substrate or other structure loaded into or placed in the chamber. The silicon carbide builds up as a shell or a deposit on the substrate. Different articles may require different thicknesses, and thicknesses can range from less than 100 microns to over an inch or two inches thick. The thickness can be controlled by controlling the deposition time and/or other process variables. When the desired deposition thickness is reached, the mandrel or substrate is then removed from the deposition chamber and the deposited silicon carbide is separated therefrom.
In one method for forming SiC articles, methyltrichlorosilane (CH3SiCl3 or MTS), hydrogen (H2), and argon (Ar) gases are introduced into the furnace through an injector. MTS is a liquid at ambient or room temperature, and sufficient vapor can be delivered into the reaction vessel by feeding carrier gas through the MTS liquid or by picking up vapor above the liquid. Gases that are unreacted in the furnace are pumped out by a vacuum pump, filtered, and cleaned in a gas scrubber before being vented to the atmosphere.
Some industries require thinly-formed silicon carbide rings or articles. The current technology of producing monolithic ceramic parts via the chemical vapor deposition process includes producing large sheets of the ceramic material in a CVD furnace. SiC is deposited onto a flat or box-shaped substrate to form the relatively large, flat sheets, from which the final ceramic part or parts are machined. The machining process includes cutting the rough shape out of the large sheet, grinding the piece to near the desired thickness to produce a blank that approximates the final form but with surplus material thickness on each face, and then machining the blank to the dimensions of the final form.
When CVD materials are deposited on large flat substrates, e.g. plates, the material exhibits a direction of crystal growth that is perpendicular to the plane of the flat substrate. The material is not necessarily deposited evenly to form a sheet that is uniform in thickness or in microstructure, so that there may be thicker portions in some areas and thinner portions in other areas, instead of a uniform thickness throughout. Because the as-deposited sheet typically has a thickness profile that is non-uniform, the thickness of rings in rough shape form can vary significantly, depending upon where they were cut from the sheet. Moreover, there may be significant thickness variation within an individual ring in rough shape form.
In addition, due to the large size of the SiC sheet from which the SiC rings are typically formed, different microstructures may exist at different areas of the sheet. For example, when a ring is produced from a sheet of deposited material, differences in the material characteristics across the thickness of the deposited sheet may lead to increased tension or stress within the material that can cause a slight axial bow or curve in the machined ring.
Another manifestation of the different microstructures across the large plate is a variation in cosmetic appearance across the final machined parts, e.g. rings. These variations in cosmetic appearance can be further magnified when the finished parts are coated with other vacuum deposited or vapor deposited coatings, especially CVD deposited coatings such as silicon.
Additionally, because of the thickness variations, the CVD process time must be increased to bring the low deposition rate areas up to the minimum thickness requirements for the desired parts. The higher deposition rate areas then cause the pieces cut from the sheet to require more machining time in order to be ground to the required thickness.
Moreover, there is limited flexibility in the geometry of a sheet and the pieces to be cut, so there is material waste due to the layout pattern of the pieces. There is also surplus material in the piece pattern that is cut or ground from between and within the pieces to make the blanks, such as edges and centers of rings, which results in a large quantity of scrap material. For example, the material that is cut out around each ring and between each ring (to form an inner diameter of the ring) is typically wasted, much like the unused cookie dough remaining after cookies are stamped out from a roll of cookie dough.
There are also occasional problems with cracking of a large, CVD-produced plates or sheets of ceramic material during the CVD process, which can reduce the yield significantly. The combination of cracks and the scrap material from the sheet and the material that is ground from the blanks lowers the average raw material-to-product conversion ratio significantly.
An alternate method that has been used to manufacture silicon carbide rings includes mounting disk-like mandrel substrates through their respective centers in a spaced and parallel relationship in grooves on a shaft. The planar surfaces of the disk-like mandrel substrates are oriented perpendicular to the axis of the shaft. During processing, the shaft is rotated as gases are injected into the chamber, such that ring-shaped ceramic parts are formed by deposition on the mandrel substrates.
A different but related method of creating silicon carbide rings is to suspend within a deposition chamber individual, flat graphite ring mandrels having an outside diameter and an inside diameter similar to those desired in the SiC rings. The gas mixture of MTS in hydrogen and argon is fed into the chamber and silicon carbide is deposited on the mandrels to form rings. Once the rings are removed from the graphite ring mandrels, their inner and outer diameter can be machined to the desired dimensions.
One problem related to forming silicon carbide rings from these alternative processes is that the mandrels need to be rotated throughout the formation process to prevent build-up in undesired areas. In many instances, the gases are injected into the reaction chamber such that the gases are not focused on any particular mandrel or surface of interest, but instead are allowed to deposit non-uniformly on all surfaces of the reactor. The rotation or suspension of the substrate in the reaction gas stream is intended to prevent or limit this non-uniformity of deposition.
The specifically-shaped mandrels are also complicated to manufacture. In the embodiment that includes a shaft with disk-shaped mandrels placed in grooves, the grooves must be specially machined for receiving and supporting the disk-shaped mandrel substrates. In the graphite ring embodiment, each ring support axis must have protrusions or tabs that facilitate their suspension or placement in the chamber, and material is often deposited in or around the protrusions or tabs.
Additionally, in each of the above-described prior art methods, the resulting ring has a crystal growth that is oriented axially relative to the ring or finished article, not radially oriented relative to the ring or article. In other words, in each of the aforementioned prior art methods, the microstructure contains grains having their long direction oriented perpendicular to the plane of the finished part. Further, it is known that as materials are deposited by chemical vapor deposition the size of grains increases as the growth proceeds away from the substrate. The evolution in grain structure from small grains to large grains occurs as grains with lower energy crystallographic orientations grow faster than grains with less preferred orientations, under the particular deposition conditions used. This evolution of grain structure typically produces a gradient in microstructure across the thickness of the material, which in turn causes a gradient in internal stress of the deposited material, resulting in bending or “bow” of the deposited material when it is released from the substrate. This gradient in material microstructure and stress complicates the machining process, and remains in the material even after machining is completed, often resulting in some bow or waviness in the finished part. This bow or waviness is undesirable, especially in parts that require precise tolerances and extremely consistent flatness, such as rings for use in contact with semiconductor wafers. The larger the part, the more significant the problem can become.
Regardless of these difficulties and costs associated with manufacturing silicon carbide articles, silicon carbide has a unique combination of properties that make it a particularly suitable material for a variety of applications in the semiconductor, optical, electronic and chemical processing fields. Silicon carbide articles produced by CVD processing are recognized to exhibit superior chemical, mechanical, thermal, physical and optical properties.
Some semiconductor processing apparatus, such as rapid thermal processing chambers, require the use of thin, SiC-edge rings to support Si wafers during high temperature processing. It is important that these rings be opaque to light, despite being relatively thin, in order to avoid causing irregularities in optical pyrometry temperature measurement of the wafers. Typically, CVD SiC rings are coated with a layer of CVD Si that is 50-100 μm thick to provide the ring with adequate opacity. There is thus a need in the art for a CVD SiC edge ring material that has an optical density closer to single-crystal Si than is possible with standard sheet-form CVD SiC, in order to reduce or eliminate the need for Si coating.
Accordingly, the present inventors have found a way to improve CVD processing for producing articles, in particular planar articles such as rings and discs, and more particularly silicon carbide ring-shaped articles. The resulting articles have a unique microstructural orientation relative to the shape of the article. The invention may also be used to create articles having other shapes. Such articles can be used in fixtures to support silicon and other wafers for processing, susceptor rings for supporting wafers in semiconductor furnaces, and as wafer edge rings.